Generally, gear mechanisms, such as a trapezoidal thread worm gear mechanism or a rack and pinion gear mechanism, are used as a mechanism to convert rotary motion of an electric motor to axial linear motion in an electric linear actuator used in various kinds of driving sections. These motion converting mechanisms involve sliding contact portions. Thus, power loss is increased and, accordingly, size of the electric motor and power consumption are increased. Thus, ball screw mechanisms have been widely used as more efficient actuators.
In prior art electric linear actuator, an output member can be axially displaced by rotating a nut forming a ball screw by an electric motor supported on a housing of the actuator. A ball screw shaft, inserted into the nut, is axially driven. As usual, the friction of the ball screw mechanism is very low. The nut can be easily rotated by a thrust load applied to the output-side member. Thus, it is necessary to hold the position of the output member when the electric motor is stopped.
Accordingly, an electric linear actuator has been developed where a brake means for the electric motor or a low efficient means such as a worm gear is provided as the power transmitting means. One representative example of the electric linear actuator is shown in FIG. 6. This electric linear actuator 50 has an actuator main body 52 with a ball screw 51 to convert rotational motion to linear motion. A gear reduction mechanism 54 transmits the rotational motion of the motor 53 to the actuator main body 52. A position holding mechanism 56 holds the position of the actuator main body 52, while engaging a first gear 55 to form the gear reduction mechanism 54.
The ball screw 51 has a screw shaft 57 as an output shaft. The screw shaft 57 is formed on its outer circumference with a helical screw groove 57a. A nut 58, formed on its inner circumference with a helical screw groove 58a, is adapted to be inserted onto the screw shaft 57. A large number of balls 59 are rollably contained in a rolling passage formed between the screw grooves 57a, 58a, arranged opposite to each other.
In the actuator main body 52, the nut 58 is rotationally supported by a pair of ball bearings 61 and 62. The screw shaft 57 is axially movably supported but is non-rotational relative to the housing 60. The screw shaft 57 can be linearly moved when the nut 58 is rotated via the gear reduction mechanism 54.
The gear reduction mechanism 54 has a first gear 55, formed as a smaller spur gear secured on a motor shaft 53a of the electric motor 53. A second gear 63, formed as a larger spur gear, is integrally formed on the outer circumference of the nut 58.
The position holding mechanism 56, with a shaft 64, acts as a lock member. The shaft 64 is able to engage the first gear 55. A solenoid 65, acting as a driving means for driving the shaft 64, engages and disengages the shaft 64 with the first gear 55. The shaft 64 is made as a bar member and is linearly driven by the solenoid 65. The tip end of the shaft 64 is adapted to engage a receiving part 66. By actuating the solenoid 65, rotation of the first gear 55 can be firmly stopped by the shaft 64 engaging the first gear 55. Thus, it is possible to stably hold the position of the screw shaft 57 in the actuator main body 52 without any slippage therebetween. See, JP2009-156416 A.
In the prior art electric linear actuator 50, the gear reduction mechanism 54 and the ball screw 51 are contained within a two-piece type housing 60. The rotational power of the electric motor 53, mounted outside the housing 60, is transmitted to the ball screw 51, via the gear reduction mechanism 54. The nut 58 converts the rotational power of the electric motor 53 to the linear motion of the screw shaft 57.
In this kind of automobile, electric linear actuator 50, driving parts, such as the ball screw 51, are contained within the housing 60 to prevent entry of muddy water or oil and the scattering of grease contained in the housing 60. Accordingly, the housing 60 should withstand a load received from the driving parts.
For example, the housing is required not only to have sufficient strength, not to be damaged when the screw shaft 57 collides with the bottom of the housing 60, but also to have desirable strength and durability not to cause deformation of fitting surfaces of the housing 60 and adverse influence to sealability.